In the medical field, for example, the tubular form is common to a variety of medical devices. Graduation scales, warning messages and other information often must be printed on syringe barrels, pipettes, pipette tips, needle shields and the like. Since these devices commonly are made of polypropylene, ink will not adequately adhere without prior surface treatment of the device. Moreover, since the devices are cylindrical, round, or otherwise multi-surfaced, the entire circumferential or perimeter surface must be treated. Because of the large numbers of such devices which commonly are manufactured in commercial production, it also is necessary that such treatment be carried out on a continuous and substantially automated basis.
It is known to treat the surfaces of such plastic devices by plasma existing in the high impedance, high frequency arc discharge generated between two electrodes located on opposite sides of the treated surface. One form of such surface treatment apparatus is disclosed in U.S. Pat. No. 4,724,508 wherein formed plastic articles are successively directed between a plurality of pairs of specifically configured insulated electrodes. A peripheral surface of the article directed between each pair of electrodes is circumscribed by a charged field which causes a chemical modification of the surface. The use of bare metal electrodes, however advantageous, is not possible in such surface treatment devices because the high temperature spark propagated between the two electrodes will cause the article to melt. Special insulation on one or both electrodes typically is used to moderate the arc and prevent overheating and melting of the plastic material. Quartz glass typically is used as the insulator for the electrode because it is the only effective insulative material that can be economically formed about electrodes of such shape. Stress concentrations in the curved corners of the electrodes result in higher operating temperatures and ultimate periodic cracking and failure, which in turn creates costly down time in the operation of the equipment. In order to reduce such overheating and failure of the electrode insulation in devices of such type, provision must be made for directing cooling air across the electrodes, which increases the complexity and cost of the equipment. Even then, insulation failure is common in such devices. In addition, the direction of cooling air across the electrodes tends to force toxic ozone out of the treatment zone, and in order to prevent such occurrence and to satisfy OSHA requirements, relatively large capacity exhaust equipment must be employed to remove the ozone, further increasing the cost of the equipment.
Another known method for treating surfaces of formed plastic articles utilizes a plurality of baremetal, discharge electrodes each disposed in close proximity to a metal electrode formed as a plate or other generally flat shaped surface. The discharge electrodes are aligned in spaced relationship to the flat electrode so that a zone of treatment is established therebetween. The flat electrode is covered with a dielectric material such as plastic, quartz, ceramic or silicone to prevent sparking or high temperature arcing between the electrodes which would cause melting of the treated articles. Such insulation may have a flat shape and hence is relatively easy to manufacture. As each article is passed through the treatment zone defined by the discharge electrode and the flat electrode, a peripheral surface of the article is partially circumscribed by the discharge with resulting molecular modification.
A problem with such bare discharge electrode surface treatment systems is that the discharge does not fully cover the entire circumferential surface of the articles being treated. Treatment of the complete circumference of a cylindrical device necessitates rotating the device 180 degrees and redirecting it through the treatment zone, which requires additional costly handling of the parts and limits the production capacity of the treatment system. Attempts to overcome such problems by increasing the intensity of the discharge has led to excessive heat generation and melting of the treated items, especially when the speed of travel through the treatment zone should be slowed or interrupted. While intensity of the discharge can be controlled by insulation, the thicker the insulation the more difficult and costly it is to manufacture. Moreover, dielectrics with good heat dissipation characteristics and with sufficient dielectrics strength against high voltage breakdown, such as silicone, quartz glass and ceramic, all have high dielectric constants, and hence, may not sufficiently reduce the intensity of the charge. Furthermore, the discharge from such bare metal discharge electrodes is difficult to control. As the electrodes are energized, the discharging arc progresses to relatively high intensity upon a relatively small increase in operating voltage. The discharges of adjacent bare discharge electrodes also tend to compete with each other, altering the discharges of the electrodes and further making it difficult to control the intensity and uniformity of the electrical field in the treatment zone.